Ambigrams are words that can be read from multiple angles, generally they can be flipped and read as the same or another word. They may not really be type in the same way as a typeface, but because each letter is designed as part of the specific ambigram, they share some interesting similarities with graffiti pieces. As in a piece, the letters must be consistent and work in relation to each other only within the word they form, rather than as an entire alphabet. While I’m not aware of any graffiti ambigrams, it seems like there is the potential for them.
John Langdon is one of the most well known creators of ambigrams, and his site has some great examples. http://www.johnlangdon.net/ambigrams/